PTR records are for reverse DNS and were originally designed for troubleshooting purposes.
Back in the good ol' days, normally there was a 1:1 mapping between a hostname and an IP address. With the invention of HTTP/1.1 and Host headers, virtual hosting support in mail servers, web servers etc. this is normally not as black and white.
However, being able to do a simple "host 18.104.22.168" on your IP address of choice and know what the hostname of that device is and subsequently derive what function it carriers out within your network is important. Especially for network related services (routers, mail servers, DNS servers etc.)
The main original reason for this is troubleshooting, it became used in the fight against spam as spammers tended to use poorly configured networks or home machines without reverse DNS records.
Lack of PTR records generally signifies a poorly configured network. Add in PTR records, as there are pieces of software that sometimes assume they exist and they simply make your life easier.